1. Field of the Invention
The invention relates generally to an axially movable switching sealing ring within a groove, and more specifically to an improved form of such a switching ring for use in the piston of a piston-cylinder unit to control fluid flow across the piston.
2. Background
U.S. Pat. No. 4,595,182 discloses a piston-cylinder unit in the form of a gas spring, which has an axially movable switching sealing ring within a groove on the piston. The switching sealing ring is rectangular in transverse cross section. Owing to frictional force between the ring and the inner wall of the cylinder, the ring is able to move between the lateral faces of the groove, since the groove has an axial width greater than the axial width of the ring. Fluid flow through the piston is thereby controlled in both directions of axial movement of the piston.
The switching sealing ring is cut in sections from a cylindrical blank. In this cutting operation, it can hardly be avoided that the sections turn out to be more or less skewed, e.g., the axially opposed (upper and lower) sides are out of parallel. When the switching sealing ring is assembled, it cannot readily be checked how the surfaces of the opposite sides are positioned with respect to one another. Extensive testing has revealed that the described skewedness of the upper and lower sides of the ring significantly influences operating behavior. The upper and the lower sides of the switching sealing ring are acted upon by pressure within the gas spring. When the geometries of the upper and lower sides of the ring are undefined, it may happen that in an application according to U.S. Pat. No. 4,595,182 the switching sealing ring executes an axial motion more rapidly or more precisely in one direction than the other, which can be detected when the unit is tested.
To remedy the problem, it has been proposed to produce the switching sealing ring in an injection molding process. As a rule, switching sealing rings that are produced by this process clearly are more accurate in shape, but on the other hand are also much more costly to manufacture. In a mass-produced product such as, for example, a gas spring, such a higher price is unacceptable.